It is well known that for many technical applications it is necessary to have rechargeable power sources of high specific power (over 0.5 kW/kg) at a rather high specific energy (over 1 kJ/kg). Widespread accumulators of various types have high specific energy (100 kJ/kg and higher) but they are not able to provide high specific power, because they possess too high internal resistance (M. A. Fetcenco et al. In 16th International seminar and exhibit on primary and secondary batteries. Mar. 1-4, 1999, Florida, USA).
Conventional capacitors (oxide-electrolytic, oxide-semiconductor and ferroelectric ones) possess high specific power (10 kW/kg and higher) but low specific energy (less than 0.5 kJ/kg) (D. Evans. The 9th International seminar on double layer capacitors and similar energy storage devices. Dec. 6-8, 1999, Florida, USA).
Combination of high specific power with relatively high specific energy is attained in special electrochemical energy storage devices, for example, in electrochemical “double-layer” capacitors, where energy is accumulated in the form of electrostatic energy of double electrical layer on the boundary between “electrode (electron conductor) and electrolyte (ion conductor)” (N. S. Lidorenko. Reports Acad. Sci. USSR, 1974, vol. 126, p. 1261), in accumulators of special design, characterized with diminished electrode thickness (M. A. Fetcenco et al. In 16th International seminar and exhibit on primary and secondary batteries. Mar. 1-4, 1999, Florida, USA), as well as in hybrid electrochemical capacitors (RU, C1, 2145132),where one electrode accumulates energy in the form of electrostatic charge of double electrical layer, like in electrochemical double-layer capacitors, and another one—in the form of internal energy of electrochemical reactions products, like in accumulators.
Electrochemical double-layer capacitors (N. S. Lidorenko. Reports Acad. Sci. USSR, 1974, vol. 126, p. 1261) have positive and negative electrodes made as a rule of carbon materials with highly developed surface accumulating energy in the form of double-layer charge. Stored specific energy can be calculated by the formula used for any capacitor:Espmax=C·U2/2m,  (1)where Esp—specific energy per mass unit,
C—capacitance of the capacitor,
U—operation voltage,
m—mass.
Maximum (peak) specific power of a capacitor is determined with the following formula:Pspmax=U2/4m·Ri,  (2)where Ri—equivalent internal resistance of the capacitor.
From formulae (1) and (2) it follows that increase in specific energy and specific power of electrochemical double-layer capacitors (at one and the same mass) is possible by means of increasing operation voltage, increasing specific capacitance and decreasing internal resistance.
Increase in operation voltage of electrochemical double-layer capacitors is achieved e.g. by going over to anhydrous organic electrolytes with decomposition voltage over 3 V. However, in this case internal resistance Ri grows, i.e. power decreases. Besides, anhydrous electrolytes are expensive, often toxic, fire hazardous and explosive.
Nevertheless, electrochemical double-layer capacitors with anhydrous electrolytes find an application, achieving in their best samples high enough characteristics: Espmax≈10 J/g, Pspmax≈3.5 W/g and service life more than one hundred thousand cycles of recharge. Though high cost, fire and explosion hazards are main drawbacks limiting possibilities for use of these capacitors.
Accumulators of special design (M. A. Fetcenco et al. In 16th International seminar and exhibit on primary and secondary batteries. Mar. 1-4, 1999, Florida, USA), characterized with diminished thickness of electrodes, have very high values of specific energy (over 20 J/g), are not expensive, use non-volatile and fire safe aqueous electrolyte, but they have comparatively low power (Pspmax<1W/g) and limited service life—at most ten thousand cycles of recharge.
The stored specific energy of an accumulator can be calculated by the following formula:Espmax=q0.U/m  (3)where q0—full charge of the accumulator at discharging by very small current.
At increase in discharge current the charge decreases, accumulator voltage decreases both at the first moment and during discharging, at first slowly, then rapidly. Usually a quick voltage drop cannot be tolerated at operation of an accumulator because of unfavourable effect on service life.
Specific energy Esp, released by accumulator at discharge, as well as its specific power Psp, depend on discharge current I:Esp=q(I).Uav(I)/m,  (4)Psp=I.Uav(I)/m,  (5)where q(I)—charge,
Uav(I)—average discharge voltage.
To achieve high values of specific power Psp it is necessary to have high ratios I/m, i.e. high current values per mass unit of the accumulator. It is just this reason that explains design peculiarity of high-power accumulator electrodes: very small thickness of both current-carrying collectors and active material.
In hybrid electrochemical capacitors (RU, C1, 2145132) one electrode (usually negative one) operates on the principle of double-layer capacitor, the other (usually positive one)—on the principle of accumulator, therewith aqueous solution of electrolyte is used in the capacitors.
Change of voltage during discharging of a hybrid electrochemical capacitor takes place mainly due to discharging of double-layer carbon electrode while potential of “accumulator” electrode changes relatively weakly. Internal resistance Ri depends on both electrodes since redox reactions proceed with over-voltage.
Due to the above circumstances, hybrid electrochemical capacitors have discharge characteristic similar to that of capacitors and their specific energy and power are determined with formulae (1)-(2). Hybrid electrochemical capacitors occupy an intermediate position between electrochemical double-layer capacitors and accumulators, they have high specific power (Pspmax≈3,5 W/g) and energy (Espmax≈10 J/g), they are much cheaper than double-layer capacitors with organic electrolyte, they are fire- and explosion-proof. Service life of a hybrid electrochemical capacitor is determined by the positive electrode and, since the discharging charge is usually several times less than its full charge, the number of recharge cycles may be as high as 50-100 thousand cycles. However, due to high price of high-quality carbon material used in negative electrodes price of hybrid electrochemical capacitors is in general higher than that of accumulators.
Positive and negative electrodes for electrochemical energy storage device of high specific power are known each of which is made in the form of backing carrying on one or both sides active element interacting with aqueous alkaline electrolyte of the electrochemical energy storage device in the process of redox reactions of charge/discharge (RU, C1, 2121728).
The backing is made out of electron-conductive but not ion-conductive material that is chemically and electrochemically non-active in the working electrolyte of the electrochemical energy storage device and functions in the electrode simultaneously as a carrying base and as a current lead to the active element.
The active element is structurally formed on the backing by means of applying a coating of a material of initial composition including basic metals out of a certain group or their alloy, or an alloy of at least one metal out of this group with one or several metals-modifiers out of the group: copper, lanthanum or lanthanides, molybdenum, tungsten, manganese, vanadium, titanium, tin, lead, bismuth, gallium; pore-forming metals out of the group: aluminium, zinc, alkali and alkali-earth metals or their combinations with further chemical and/or electrochemical treatment of the coating in solutions of acids, salts or alkalis. Group of basic metals for positive electrodes: iron, nickel, cobalt, silver; for negative electrode: iron, nickel, cobalt, cadmium. As a result of this treatment there are formed at the same time highly-developed surface of the coating (due to etching out of pore-forming metals) and thin oxide and/or hydroxide film of active material on the coating surface—the film made of mono- or polymolecular compounds on the interphase boundary “electrode-electrolyte”. Thus, the formed active element constitutes a highly-porous electron-conductive layer with large true surface area coated with electron-nonconductive oxide and/or hydroxide film. The said film and the porous coating on which the film is located form two functionally and structurally independent components (phases) of active element, the first phase functioning as active material and the second phase—as current-carrying collector. Total current supply in the electrode is carried out through the backing.
The said technical concepts are taken as a prototype for the first and second embodiments of the present invention.
The described design of electrodes of an electrochemical energy storage device in which the active material of the active element (thin oxide and/or hydroxide film) is located on the developed surface of the current-carrying collector (a highly-porous layer of coating on the backing) realizes the traditional principle of mutual arrangement of the main phases participating in current-producing reactions of electrode in the electrochemical energy storage device, namely “electron conductor (collector)—active material (oxides, hydroxides)—electrolyte”. Due to extremely small thickness of oxide and/or hydroxide film of active material electrochemical reactions of charge-discharge proceed with a high rate which determines high operating characteristics of the electrochemical energy storage device.
Common drawback of the known positive and negative electrodes for electrochemical energy storage device of high specific power is insufficient service life—at most 10,000 cycles of recharge. Besides, maximum specific characteristics of negative electrode are realized when cadmium is used as the base metal, which is an environmental hazardous material.
Another electrochemical energy storage device of high specific power is known in electrodes of which the described traditional concept of mutual arrangement of main phases involved in current producing reactions is realized, comprising at least one negative and one positive electrodes submerged in aqueous alkaline electrolyte and divided by a separator—a layer of an ion-conductive but not electron-conductive material. Each of the electrodes comprises an active element interacting with electrolyte—electron-conductive coating applied on the backing, on the developed surface of which a thin oxide and/or hydroxide active material film is formed taking part in charge-discharge redox reactions of the electrode at operation of the energy storage device. Therewith, the positive and negative electrodes differ by their basic metals being part of coating applied on the backing. For positive electrode these are metals of the group: iron, nickel, cobalt, silver, or their alloys, for negative electrode—metals of the group: iron, nickel, cobalt, cadmium or their alloys (RU, C1, 2121728).
This technical concept is taken as a prototype for third, fourth and fifth embodiments of the present invention.
Discharge characteristic of the electrochemical energy storage device by its shape lies between discharge characteristics of a capacitor and an accumulator, but more close to the latter (Example 5, FIG. 6). At discharge current I=0.5 A the electrochemical energy storage device discharges during about 2.5 seconds at average voltage of about 1 V, then voltage quickly drops. It means that charge q (0.5)=0.5□2.5=1.25 C, Uav=1 V. Calculation of electrodes and separator mass based on data of examples 3-5 gives the following: mass of negative electrode is 60 mg, mass of positive electrode is 150 mg, mass of a separator impregnated with electrolyte is approximately 17 mg, total mass being 227 mg. Calculation made according to formulae (4), (5) leads to the following values: Esp=5,5 J/g, Psp=2,23 W/g.
This shows that in the known electrochemical energy storage device of high specific power the problem of enhancement of specific electrical characteristics is successfully solved at acceptable cost of the device due to use of electrodes of a certain design. Characteristics of specific energy and power achieved in the prototype are well on the level of the best world technology. Thus the electrochemical energy storage device taken as a prototype can compete both with double-layer and hybrid electrochemical capacitors by specific energy and power gaining in price.
Drawback of the known electrochemical energy storage device of high specific power is insufficient service life—at most 10,000 cycles of recharge. Besides, maximum specific characteristics are realized in the known electrochemical energy storage device when cadmium is used as the base metal, which is an environmental hazardous material.